Suspect IP address has been hacked and another party is using it.

I suspect that my IP address has been hacked by an outside party and they are using it to download copyritten content.  I was recently notified by Xfinity that my IP address was used the download a game without authorization.  We don't play any video games in our house and we have no children home who could.  What can we do?  This was very unsettling.